Described in 1838 by French scientist Anselme Payen, cellulose has the molecular formula (C6H10O5)n. It is the most abundant organic polymer, being used in an amount of about 1.5×1012 tons per year. It has been used as a renewable, biodegradable and environmentally benign chemical raw material for 150 years. Cellulose is semicrystalline, having both crystalline and amorphous regions. It is densely packed with strong inter- and intramolecular hydrogen bonds conferring excellent mechanical properties.
Cellulose nanocrystals (CNCs) have emerged as a new class of nanomaterials for polymer reinforcement and nanocomposite formulation owing to their exceptionally high mechanical strength (modulus of 145 GPa; Marks, 1967), tensile strength of 7.5 GPa ({hacek over (S)}turcová, 2005), chemical tunability, and anticipated low cost. CNCs have also been fostered for diversified applications including enzyme immobilization (Mahmoud et al., 2009), drug delivery, and biomedical applications (Dong and Roman, 2007).
In order to produce CNCs, fiber sources from various vegetative wastes with high initial cellulose contents are being considered as potential starting materials due to their low costs. The amorphous regions of the cellulose fibers must be chemically removed to yield highly crystalline CNCs. Popular acid hydrolysis using a single concentrated acid or an acid mixture, often with the aid of an oxidant, is capable of dissolving the amorphous regions (Revol et al., 1992), leaving behind CNCs with crystalline rod-like fibers. Such procedures, however, are expensive, requiring considerably high initial capital investment and having high operating costs due to the corrosiveness, safety issues and hazardous waste treatment/disposal requirements of such acids and their by-products. Additional pre- and/or post-treatment steps with alkaline or bleaching reagents are required to remove non-cellulosic fiber contents (e.g. lignin, pectin, hemicelluloses, etc).
Bai et al. (2009) describe a method for the production of CNCs with narrow distribution from microcrystalline cellulose (MCC). A conventional sulfuric acid procedure was used to produce CNCs (Dong et al., 1998). This process is known to produce a wide range of size distribution. In order to obtain a narrow size distribution of CNCs, differential centrifugation with at least six cycles was required. Even so, the CNCs still exhibited at least four different aspect ratios.
US 2008/0108772 (Oksman et al., 2008) describes a process for producing cellulose nano whiskers by treating MCC with HCl, as well as a new extrusion method to produce a reinforced organic polymeric material. The production of cellulose nano whiskers using HCl hydrolysis required pure cellulosic materials (e.g. MCC) and the resulting cellulose nano whiskers had a large size distribution. Fractions of cellulose crystals with larger size were isolated by centrifugation at low speed and discarded. The cellulose nano whiskers produced had a large size distribution of 100 nm to 1000 nm in length and 5 nm to 15 nm in width.
Persulfates, for example ammonium persulfate, are well known strong oxidants. In the prior art, for example as described in U.S. Pat. No. 5,004,523 (Springer and Minor, 1991), ammonium persulfate has been used for the isolation of lignin from lignocellulosic materials. Usually a mixture of ammonium persulfate together with either strong acid (50% HCl or H2SO4) or strong base (KOH or NaOH) is required. The product isolated from this process is bleached cellulose, not CNCs.
There remains a need for a simple, cost-effective process for producing CNCs, especially from vegetative biomass.